Female sexual attraction to male urine. (A) Total time spent under urine stimuli from males (blue bars) and from matched females (pink bars), together with the difference in time spent under male minus female stimulus (circles), plotted as means ± standard error of mean. Significant P values indicate greater attraction to the male stimulus (matched pair t-tests of log transformed data (t) or Wilcoxon matched pair tests (z) when transformed data did not approximate normality). Wild: urine from a random selection of wild males and females (n = 15); B6: normal intact B6 urine (n = 12); low molecular weight (LMW) urine fraction (< 3 kDa, n = 12): high molecular weight (HMW) urine fraction (≥ 3 kDa, n = 12); equal protein: male urine diluted to the same protein concentration as female urine (n = 12); water (open bars): two water stimuli (n = 10). The breakdown of time sniffing each stimulus and time under each stimulus not sniffing is shown in Figure S1. (B) Intact urine, HMW and LMW fractions from C57BL/6 (B6) male and female mice resolved by 15% one dimensional SDS-PAGE. Major urinary proteins (MUPs) were the only major protein bands observed, including a male-specific 18893Da MUP (darcin) which shows unusually high mobility for its size on reducing SDS-PAGE and appears as a band equivalent to 16 kDa .